A tablet (iPad or Android) game which is designed to be used while exercising.

Use ANT or Bluetooth 4.0LE to tie into a treadmill, bike, or other exercise equipment to get output measurements (speed); ideally, find some devices which allow two way commands (not common at all right now).

Networked games against other people, or vs. computer or past personal performance. The interesting part is a "use while running" interface for the touchscreen, requiring inputs (using gross motor skills, not fine control) to do things in-game while retaining performance. Or maybe use audio output for instructions (i.e. "press the blue button, then the red, then the green" while keeping heart BPM above 130, and targets moving on screen.